1. Field
The present disclosure relates to cathode active material for lithium batteries and a method of manufacturing the same, and more specifically relates to cathode active material for manufacturing lithium batteries which is doped by different metal and has gradient concentration and a method of manufacturing the same.
2. Description of Related Art
Recently, as utilization of portable electronic appliances such as camcorders, mobile phones, notebook PCs are generalized by rapid development of electronic, communication and computer industries, requirement for light batteries with long life and high reliability is elevated. Particularly, the requirement of the lithium secondary battery are increased day by day as power source for driving these portable electronic information communication devices because the lithium secondary batteries have driving voltage over 3.7 V and energy density per unit weight higher than nickel-cadmium batteries or nickel-hydrogen batteries.
Recently, studies about power sources for electric vehicles in hybrid an internal combustion engine and the lithium secondary battery are lively progressed in America, Japan, Europe and etc. A development for P-HEV (Plugin Hybrid Electric Vehicle) battery used for vehicles capable of less than 60 mile distance covered in a day are lively progressed around America. The P-HEV battery has characteristics little short of electric vehicle thereby the greatest problem is development of high capacity battery. Particularly, the greatest problem is development of a cathode material having high tab density over 2.0 g/cc and high capacity property over 230 mAh/g.
Cathode materials in common use or development are LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFePO4 and etc. LiCoO2 is a material having stable charge/discharge characteristics, superior electron conductivity, high battery voltage, high stability and flat discharge voltage property. However, cobalt (Co) is rare in deposits and expensive, in addition that, it has toxicity to human thereby requiring development for other cathode materials. Further, these have weakness of deteriorated thermal property because crystal structure is unstable by delithiation in charging.
To improve these problems, a lot of attempts in which transition metal element replaces for a part of nickel are trying in order to shift heat generation starting temperature to high temperature portion or make heat peak broaden for preventing rapid heat generation. However, satisfaction has not been acquired yet.
In other words, LiNi1-xCoxO2 (x=0.1−0.3) material in which cobalt substitutes for a portion of nickel shows superior charge/discharge property and cycle life characteristics, however, thermal stability problem is not solved. In addition, Europe Patent No. 0872450 discloses LiaCobMncMdNi1−(+c+d)O2 (M=B, Al, Si. Fe, Cr, Cu, Zn, W, Ti and Ga) type in which another metal as well as cobalt and manganese substitute for nickel locations, however, thermal stability problem is also not solved
To remove these weak points, Korea Patent Publication No. 10-2005-0083869 suggests lithium transition metal oxide showing concentration gradient of metal composition. In this method, interior material of predetermined composition is synthesized and coated by a material with different composition to be double layer followed by mixing with lithium salt and performing thermal treatment. Lithium transition metal oxide which is commercially available may be used as the interior material. However, this method has a problem of unstable interior structure because metal composition of cathode active material between the inner material and outer material is discontinuously changed. Further, Ni content in transition metal should be increased for acquiring high capacitance in cathode active material of layered system, however structural stability in charging is relatively lower than other transition metals such as manganese and oxygen and electrolyte effluent from electrode material are reacted each other and high exothermic reaction is occurred at relative low temperature when battery temperature are elevated by internal short if nickel content is high, thereby existing dangerous of battery burning and blowout.